Vascular stents are commonly used for the treatment of various vessel diseases. They are implanted transluminally using a catheter delivery system, to advance them to a stenting site, where they expand in order to widen the clogged or narrowed blood vessel. During the implantation procedure, the exact position of the stent within the vessel is monitored, mainly using X-ray imaging techniques. In order to ensure accurate placement, a good visibility of the stent is crucial. In general, this is achieved by attaching suitable markers which are made from materials with a radiopacity larger than that of the stent material. In addition, these markers have to be sufficiently large to provide a good X-ray contrast. For a precise determination of the stent position during delivery, it is advantageous to place the markers at both ends of the stent.
A tubular stent with a plurality of radiopaque markers attached to selected inflection zones around its circumference is disclosed in WO 2002/015820. The markers are spoon-shaped in a way that almost a complete ring of marker material is formed in the radially compressed state of the stent, providing a particularly high level of radiopacity during transluminal delivery. Thereby, an increase in visibility to radiation is achieved without any increase in the wall thickness of the stent at the position of the markers, maintaining a large radial opening for fluid flow as well as a small cross-sectional profile. Ideally, the number of markers in such a ring of markers as described above is to be kept small, so that each marker is large enough to facilitate sufficient visibility even in the radially expanded state of the stent. Furthermore, the number of marker/inflection zone joints should be kept at a minimum in order to reduce the risk of loss of a marker in the body following failure of such a joint after the stent has been placed.
For the case of self-expansible stents, delivery to the narrowed portion of the blood vessel is performed with the use of a co-axial catheter delivery system. Hereby, the stent is kept in its radially compressed state by an outer sheath. A co-axial cylinder inside the sheath is used for displacing the stent axially, relative to the outer sheath. Once the stent has been placed at the desired position within the vessel, the outer sheath is withdrawn while the inner cylinder pushes against one end of the compressed stent, precipitating the release of the stent from the delivery system. This procedure can impose on a stent such as the one disclosed in WO 2002/015820 stresses concentrated on the radiopaque markers which protrude axially beyond the axial end of the matrix of struts of the stent annulus. This stress, concentrating at the joints between the markers and the inflection zones from which they are cantilevered, has been identified by the present inventor as a feature that can and should be reduced or even eliminated.
This patent application declares the priority of GB 0 717 481.6. In that GB priority application the search report conducted on the claims by the UK Patent Office cited the following documents: WO 2006/047977; DE 20 2004 014 789; WO 2004/028408 and EP 1 433 438. The disclosures of the first two citations are of the same invention from the same inventor.
WO 2006/047977 discloses a stent with an end annulus that carries radiopaque marker material in recesses in receiving elements of the stent metal. Thus, the marker material has exposed luminal and abluminal part-cylindrical opposed major surfaces and a thickness corresponding to that of the receiving element that surrounds the entire peripheral edge of the marker between its major surfaces. The marker has a width in the circumferential direction of the stent annulus, that is smaller than the circumferential length of the portion of the stent in which the marker is received. The marker enters the recess from a position radially outside the stent annulus, conical surface portions of the marker and the receiving portion serving to guide the marker into a snugly fitting disposition within the receiving portion, supported in all sides by the receiving portion.
The disclosure of WO 2004/028408 is similar. As to EP 1 433 438, it discloses other sorts of radiopaque markers on stents, notably a wire of radiopaque material wound around a strut of the stent, or a sleeve of radiopaque material crimped around a strut of the stent. Self-evidently, the stent is locally thicker in the radial direction, where such markers are located.